The present invention relates to electrophotographic devices such as laser printers, and in particular to the determination of media type by electrophotographic devices.
Electrophotographic processes for forming images upon print media are well known in the art. Typically, these processes include an initial step of charging a photoreceptor which may be provided in the form of a drum or continuous belt having photoconductive material. Thereafter, an electrostatic latent image is produced on the photoreceptor by exposing the charged area of the photoreceptor to a light image or scanning the charged area with a laser beam. A light-emitting diode array may be used in producing the electrostatic latent image on the photoreceptor.
Particles of toner may be applied to the photoreceptor upon which the electrostatic latent image is disposed such that the toner particles are transferred to the electrostatic latent image. Thereafter, the toner particles are transferred from the photoreceptor to the print media. This process involving the transfer of toner particles unto the media is herein referred to as image transfer process. Frequently, a fusing process follows the image transfer process and fixes the toner particles on the print media. A subsequent process may include cleaning or restoring the photoreceptor in preparation for the next printing cycle.
Two imaging parameters greatly affect the final print quality of the toner image supplied to the media. These imaging parameters are the electric field applied to the media during the image transfer process and the heat energy applied during the fusing process. The electric field applied to the media and the heat energy transferred during the fusing process, in turn, are affected by basis weight and the water content of the print media. The basis weight and the water content manifest themselves as differences in dielectric thickness, heat capacity and thermal conductivity for a given print media in a particular environment.
The optimal value of the imaging parameters applied during the image transfer process depends on the resistance and the capacitance of the print media. However, most conventional electrophotographic devices use a predetermined set of imaging parameters during the image transfer process for all print media. The failure to customize the imaging parameters to the particular print media that is used can result in less than optimal image quality. The failure to customize the imaging parameters to the resistively of print media is especially likely to result in an aesthetically displeasing output because print media range widely in resistively. For example, paper and transparencies, which are both common print media, have resistibilities that may differ by approximately six orders of magnitude. As most transfer systems are designed to handle a predetermined design range of resistance (resistance is a function of resistively and the physical dimensions), setting the imaging parameters to optimize image transfer onto paper leads to less than optimal quality output on transparencies, and vice versa.
Therefore, an electrophotographic device and method that can determine electrical properties (e.g., capacitance and resistance of print media) to produce high quality images is needed.
The present invention includes an apparatus and a method for electrophotographic imaging devices to adjust the imaging parameters to the type of print media, thereby achieving optimal print quality for all print media. According to the present invention, a set of rollers in an electrophotographic imaging device is made of conductive material, insulated from the device chassis, and connected to a monitoring circuit. The monitoring circuit includes a pulse forming circuit connected to a first roller and a sensing circuit connected to a second roller. The pulse forming circuit includes a capacitor and thus, a RC circuit forms when the media is positioned between the rollers. The pulse forming circuit applies a pulse to the media, and the sensing circuit measures the step response of the RC circuit. Based on the measured step height and the slope of the response, the resistance and the capacitance of the print media can be calculated. The resistance and the capacitance is then used to determine the optimal value of imaging parameters, such as the transfer bias voltage.
The step response is determined by sampling the response voltages from the voltage sensing circuit and using the samples to calculate the resistance and the capacitance of the print media. The optimal imaging parameters are determined either by calculation or by accessing a look-up table that contains pre-derived optimal values. Imaging parameters are then adjusted to the determined optimal values. The optimization process takes place between the time the print media passes between the first and second rollers and the time imaging occurs. Although the measurement may be accomplished with the media in motion, taking the measurements with the media in a temporarily stationary state (e.g., for 120 ms) improves the accuracy of the result. Thus, the optimization process of the present invention not only facilitates implementation by using a set of rollers that transport the print media, but also provides a way to determine and apply the optimal imaging parameters while the print media is moving through the imaging device.